Chilled water and DX systems are commonly used for air conditioning in commercial buildings. They are also used for process cooling and other numerous applications. They generally include at least one refrigeration circuit to provide chilled water or air that is then used in heat exchangers to cool air recirculating within terminal units or the air conditioning duct work within buildings.
A typical refrigeration circuit for such applications uses a circulating refrigerant fluid and operates on a vapour-compression refrigeration cycle. The refrigerant enters a compressor in a superheated vapour state and is compressed to a higher pressure, raising the saturation temperature of the refrigerant vapour at the same time. Superheated vapour from the compressor discharge outlet passes to a condenser, where it is cooled, de-superheated and the vapour then condenses to a saturated liquid and then to a sub-cooled liquid. In air conditioning systems in buildings, the heat rejected from the refrigerant as it condenses to a liquid state is usually carried away by ambient air. The sub-cooled liquid refrigerant output from the condenser is next passed through an expansion device, where it undergoes an abrupt reduction in pressure, resulting in an adiabatic flash evaporation of a proportion of the liquid refrigerant. This dramatically lowers the saturation temperature of the remaining liquid refrigerant, which now becomes a liquid and vapour mixture. The refrigerant mixture then passes to an evaporator, through which the water or air to be chilled flows. The liquid part of the cold liquid/vapour refrigerant mixture evaporates and in so doing removes heat from the circulating water or air to chill it. Superheated refrigerant vapour from the evaporator outlet then returns to the compressor inlet and the cycle starts again.
Various types of compressor have been used for refrigeration circuits, including rotary, screw, scroll, reciprocating and centrifugal compressors.
Of the available compressor types, centrifugal compressors have proved popular over many years as they operate with a simple, continuous rotary motion, with relatively few moving parts, have proved to be reliable and require little maintenance. Whilst the compression ratios achieved in known centrifugal compressors (typically about 3:1 to 4:1) are not as high as can be achieved with other types of compressors, the pressure rise and consequent temperature rises achieved are adequate for the conventional applications they are used for.
In most known centrifugal compressors, as well as other compressor types, oil is added to the recirculating refrigerant to lubricate the compressor bearings (or other moving parts). The oil must be carefully chosen so as not to react adversely with the refrigerant and must circulate freely with the refrigerant; a change in refrigerant type will usually require a change of the oil.
The refrigeration system must also be carefully designed to ensure that oil circulating with the refrigerant returns to the compressor, either by flowing around the complete system and returning by velocity entrainment, gravity and/or appropriate routing/sizing of the pipework, or an oil separator can be positioned at the outlet of the compressor and a separate return path provided for the oil to return it to the compressor from the oil separator.
More recently, to avoid the complexities associated with oil lubrication, oil-free centrifugal compressors have been proposed.
One example of an oil-free compressor is the Danfoss Turbocor™ compressor, which is a two-stage centrifugal compressor that uses magnetic bearings. These magnetic bearings require a complex control system to ensure that they operate correctly and fail-safe if there is a loss of electrical power.
US2004179947 describes another example of an oil-free centrifugal compressor, in this case using foil vapour journal bearings (sometimes referred to as “dynamic” or “hydrodynamic” gas bearings) to support the rotating shaft of the compressor, where the rotary motion of the shaft within the bearing itself generates the pressure in the vapour necessary to support the shaft. WO 00/55506 and WO 94/295597 describe further examples of centrifugal refrigerant compressors that use radial dynamic gas bearings, in which the refrigerant itself is used as the bearing fluid.
JP2004044954 describes a single stage refrigerant compressor that employs hydrostatic radial bearings, i.e. externally pressurised bearings, in which the bearing fluid is the refrigerant itself. At normal running speeds a portion of refrigerant vapour from the compressor vapour outlet is supplied to the bearings to provide the necessary pressurised flow to float the bearings. At low compressor speeds, the bearings are pressurised instead with a supply of refrigerant vapour from an accumulator. US 2009/311089 describes another example of a compressor that includes hydrostatic radial bearings.
Recently, there has been a growing desire to use the heat rejected from the refrigeration cycle of chilled water and DX systems to do some useful work, for example to provide hot water. Typical hot water temperature from a commercial/industrial boiler is 82° C. Conventional refrigeration systems, on the other hand, including those using the types of oil-free centrifugal compressors referred to above, are capable of generating hot water from rejected heat energy normally at no more than about 50-55° C. This is due to their inability to develop sufficiently high compression ratios. It would therefore be necessary to use an auxiliary heat energy source (oil or gas fired boiler) to raise the water temperature to the desired 82° C.